Align vertices into a straight line
To straighten vertices to a line:

Set pivot to 'Active Element'

Select the 2 vertices at the ends, and create a Custom Transform Orientation from them. (The little '+' in the header's Transform Orientation dropdown)


With all 3 vertices selected, and one of the ends selected last, so active, and the new transform orientation active, hit S Shift Y 0, scaling to 0 in all dimensions, except 'Y', the one along the line between the end vertices.